This invention relates to a method of operating a combustion apparatus and more particularly but not exclusively an internal combustion engine e.g. for a vehicle. Such an engine typically includes at least one combustion chamber with an inlet port for primary combustion air, means to introduce into the combustion chamber primary fuel for combustion with the primary air, an outlet port for combustion products, and an exhaust system for exhausting the combustion products to atmosphere. The primary fuel may be petrol, diesel, liquid petroleum gas, for example, or any other suitable fuel or mixture of such fuels.
The development of internal combustion engines has tended to focus on maximising the power output of the engine, achieving economy of fuel use, and the reduction of harmful emissions.
Particularly in the case of achieving reduction of harmful emissions, development has tended to concentrate on improving combustion within the engine for example by developing fuel injection systems which provide for improved fuel/air mixing in combustion chambers of the engine, and then cleaning combustion products, for example by passing the combustion products through catalytic converters for example.
The provision of catalytic converters is problematic because such devices only tend to operate to their maximum performance once very hot, such that during short journeys for example in which engines may not attain an optimum operating temperature, such catalytic converters provide substantially no beneficial effect. Moreover, such converters are expensive and require frequent replacement, as they are easily contaminated.
In previous patent application EP-A-0744006 there is proposed an apparatus for improving combustion, by introducing into the combustion chamber of the engine, a vaporised secondary hydrocarbon based high calorific value fuel, obtained by heating a mineral oil. Thus the efficiency with which the primary fuel is burned in the engine is enhanced, and this has the effect of reducing the production of harmful exhaust emissions.
In previous patent application EP-A-041831 there is described an exhaust aspirator which is adapted to permit the introduction of relatively clean air into an exhaust manifold of the engine to promote more effective exhaust gas discharge and to cause aspirated air to be drawn into a combustion chamber of the engine through an exhaust port thereof, rather than exhaust gases, during cyclic pressure changes which occur in normal engine operation.
According to a first aspect of the invention we provide a method of operating a combustion apparatus which includes at least one combustion chamber with an inlet port for primary combustion air, means to introduce into the combustion chamber primary fuel for combustion with the primary air, an exhaust port for combustion products, and an exhaust system for exhausting the combustion products to atmosphere, the method including introducing into the exhaust system secondary air, mechanically acting upon the secondary air and products of combustion in the exhaust system in the presence of a catalyst, to produce a reformed fuel, introducing the reformed fuel into the combustion chamber for combustion with primary fuel and primary air.
Preferably, the reformed fuel is introduced into the combustion chamber via the exhaust port, e.g. soon after production, but it is envisaged that the reformed fuel may be collected and stored, for later use, and may be fed with the primary fuel into the combustion chamber.
It has been found that by utilising the method of the invention, a substantial improvement in combustion efficiency can be achieved, resulting in much cleaner exhaust emissions and a significant improvement in power output. Also whereas with previous proposals for improved e.g. engine designs, it has been necessary to compromise between fuel efficiency and power output, utilising the present invention, such compromise is unnecessary.
Where the invention is applied to an engine, the method may include introducing the secondary air into the exhaust system via an exhaust aspirator e.g. similar to that disclosed in EP-A-0041831, which draws air into the exhaust system during low pressure or partial vacuum conditions occurring during the cycle of pressure changes which occur in the exhaust system during normal operation of the engine. However whereas in EP-A-0041831, the primary function of the aspirator is to damp down vacuum waves occurring in the exhaust system to improve the efficiency of exhaust gas discharge, in the method of the invention the exhaust aspirator is tuned to draw air into the exhaust system and mechanically to act upon the secondary air and products of combustion by means of pressure pulses, in a manner to optimise reformed fuel production.
Typically, an aspirator means suitable for the purpose of the method of the invention, includes a valve with a spring biased valve member, the force of the spring acting to urge the valve member into engagement with a valve seat to close the valve, and the force of the spring being overcome when sufficiently low pressure is developed in the exhaust system, to allow the secondary air to be drawn in through the valve. Tuning of such an aspirator means for the performance of the invention may involve adjusting the spring pressure so that the valve responds to a desired low pressure or partial vacuum at an appropriate time during the engine cycle, and in a manner to produce strong pressure pulses in the exhaust system adequate mechanically to act upon the combustion products and secondary air present in the exhaust system to produce reformed fuel.
It will be appreciated that in the case of an engine having a single combustion chamber, the spring may be adjusted to open the valve and allow the secondary air to be drawn into the exhaust system at commencement of the piston""s induction stroke, when a low pressure is developed in the exhaust system as a result of the (single) piston moving in the combustion chamber to induce air therein through an air inlet port, prior to the exhaust port being completely closed by an exhaust outlet valve or the piston. In a multi-combustion chamber engine, where each piston will be in a different stroke position, the valve may respond to low pressure developed in the exhaust system when one or all of the pistons are at other stroke positions.
It will be appreciated that reference in this specification to primary air being induced into the or each combustion chambers includes air being introduced into the combustion chamber or chambers by e.g. turbo or other primary air introduction means.
The aspirator may be tuned to produce from the secondary air and combustion products, which typically include un-burnt hydrocarbons, nitrous oxide gases (commonly known as xe2x80x9cNOxxe2x80x9d), carbon dioxide, carbon monoxide and water (usually in the form of steam), reformed fuel including one or both of methanol and hydrogen gas, both of which may satisfactorily provide a supplementary fuel. By operating an engine in accordance with the invention, the amount of un-burnt hydrocarbons, NOx and carbon monoxide in the exhaust gases eventually emitted from the exhaust system to atmosphere has been found to be greatly reduced, and the amount of carbon dioxide produced is significantly reduced
Preferably, the catalyst is provided by materials from which the aspirator is at least partly made, such as for example copper, but if desired Cerium may be provided in the vicinity of the aspirator which is particularly good at enhancing secondary fuel production.
Where the apparatus is an engine, the reformed fuel may be introduced into the or each combustion chamber of the engine as a result of a partial vacuum being established at the exhaust port at the end of an exhaust stroke of a piston thereof as the piston commences its next induction stroke, before the exhaust port is completely closed by an exhaust outlet valve or the piston.
Thus the reformed fuel will combust in the combustion chamber simultaneously with the primary air and primary fuel induced or injected into the engine during the next induction phase.
Where the combustion apparatus has a plurality of combustion chambers each having an exhaust port, the exhaust ports may each open to an exhaust manifold, where the aspirator or other means for introducing secondary air into the exhaust system may be located.
The method of the invention may include introducing reformed fuel obtained by mechanically acting upon drawn in secondary air and products of combustion in the exhaust system, into an inlet manifold from where air is introduced into the combustion chamber for combustion with the primary fuel.
This may be achieved by allowing reformed fuel introduced into the combustion chamber via the exhaust port while the exhaust port and inlet ports are both not fully closed, to pass out of the combustion chamber into the inlet manifold for mixing with the air or air/fuel mixture in the inlet manifold, for subsequent introduction into the combustion chamber together with primary and, and induced or injected primary fuel.
In addition to operating the apparatus in accordance with the primary aspect of the invention described above, the method of the invention may additionally include introducing into the combustion chamber of the apparatus, a vaporised secondary hydrocarbon based high calorific value fuel, obtained by heating a mineral oil, for example using an apparatus the subject of EP-A-0744006.
A typical mineral oil may for example conform to British Standard 245.
The mineral oil may be heated electrically and/or by heat exchange with hot exhaust gases produced by the engine.
It has been found that by introducing into the engine or other combustion apparatus a vaporised secondary hydrocarbon based high calorific value fuel to be burnt with the primary fuel and primary air, the engine may be operated at a lower temperature than otherwise, and as a result, the amount of particularly NOx produced during combustion, is minimised.
The combustion apparatus may be an internal combustion engine e.g. an engine in which combustion of fuel in the combustion chamber is initiated by spark, or a Diesel cycle engine in which combustion is initiated by heat caused by the compression of gases in the combustion chamber, or a spark assisted Diesel engine.
In the case of a spark ignition engine the primary fuel may be induced and/or injected into the combustion chamber together with the vaporised secondary hydrocarbon based high calorific value fuel, although where the primary fuel is introduced by injection, the vaporised secondary hydrocarbon based high calorific value fuel may be introduced with the induced air.
In each case performance of the invention has been found to improve the power output of the engine, to achieve improved economy of fuel use, and the reduction of harmful emissions.
According to a second aspect of the invention we provide a method of adapting a combustion apparatus having at least one combustion chamber, for operation in accordance with the first aspect of the invention, the method including providing the combustion apparatus with means to introduce into the exhaust system secondary air, providing means mechanically to act upon the secondary air and products of combustion in the exhaust system in the presence of a catalyst, to produce a reformed fuel, and means to introduce the secondary fuel into the combustion chamber of the apparatus via the exhaust port for combustion with primary fuel and primary air.
Thus the invention readily lends itself to retrofitting to an engine, whereas conventional catalytic converters for example are provided to cleanse exhaust emissions have to be provided as original equipment e.g. by a vehicle manufacturer to ensure that there is space provided to accommodate the converter and to ensure that the converter and engine operate satisfactorily together. However a vehicle may be adapted to perform the invention during manufacture as desired.
Where the engine to be adapted to perform the invention includes a catalytic converter already, depending on the kind of engine control employed, steps may need to be taken to ensure satisfactory operation of the engine. For example where the engine control is so called xe2x80x9copen-loopxe2x80x9d, i.e. an engine management system of the vehicle is operated substantially independently of the performance of the catalytic converter, the method of the second aspect of the invention may be performed without any additional steps being required. However where the engine control is so called xe2x80x9cclosed-loopxe2x80x9d in that an input to the engine management system is provided from a sensor associated with the catalytic converter, the method may include disabling the sensor of the catalytic converter or re-programming the engine management system so that performance of the method of the first aspect of the invention does not detrimentally affect the performance of the engine management system.
It will be appreciated that catalytic converters operate most satisfactorily with a rich fuel/air mixture being burnt in the engine. With a so called xe2x80x9cclosed-loopxe2x80x9d engine management system, the sensor associated with the catalytic converter senses the amount of un-burnt fuel and carbon monoxide, in the combustion products, and the engine management system sets the richness of the primary fuel/air mixture for optimum performance of the catalytic converter. Because utilising the present invention un-burnt hydrocarbons and carbon monoxide are used up in providing reformed fuel, an un-modified closed-loop engine management system would respond to the sensor unduly to increase the richness of the primary fuel/air mixture.